Lecture 11: Air Movement in Buildings

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1 Lecture 11: Air Movement in BuildingsNotes: __________________________________________________________________________________________________________________________________________Material prepared by GARD Analytics, Inc. and University of Illinoisat Urbana-Champaign under contract to the National Renewable EnergyLaboratory. All material Copyright U.S.D.O.E. - All rights reserved

2 Importance of this Lecture to the Simulation of BuildingsAir is critical to sustaining life and also to the thermal environment inside a building (comfort)While a zone is defined by a common air mass at a particular temperature, air is not static within a buildingUnintended flow into/out of the buildingPurposeful flow into/out of the buildingExchange between interior spacesAir movement may result in energy transfer from one area to another and thus has an energy impact on the zone and/or buildingUnderstanding the energy impact of air movement is critical to our understanding of how much energy is required to maintain thermally comfortable conditions within a space

3 Purpose of this LectureGain an understanding of how to:Define air movement between interior spacesApproximate the effect of ventilating a building (naturally or mechanically)Perform detailed air movement calculations using the EnergyPlus link to COMIS

5 Mixing Used to move air from one zone to anotherOnly has an impact on the receiving zone—user must account for energy impact on source zone through Mixing, Infiltration, etc.Can be used to set-up multiple air flow paths or a “circular” path between more than two zonesUser must specify flow rates and schedule

6 Mixing (cont’d) IDD Description: MIXING, A1 , \field Zone NameA2 , \field SCHEDULE NameN1 , \field Design Level of Air Flow in m3/sA3 , \field Source Zone NameN2 ; \field Delta Temperature in degrees Celsius“Delta Temperature” controls when mixing air from the source zone is sent to receiving zone; if positive, the temperature of zone from which air is drawn must be DT warmer than receiving zone air or no mixing occurs; if negative, the temperature of source zone must be DT cooler than receiving zone air or no mixing occurs; if DT is zero, mixing occurs regardless of the relative zone temperatures

7 Cross Mixing Used to exchange equal amount of air between two zonesHas an (equal) energy impact on both zonesOnly needs to be defined once for one of the two zonesOnly one cross mixing statement per zoneIf mixing occurs with more than one zone, must use MixingUser must specify flow rates and schedule

8 Cross Mixing (cont’d) IDD Description: CROSS MIXING,A1 , \field Zone NameA2 , \field SCHEDULE NameN1 , \field Design Level of Air Flow in m3/sA3 , \field Source Zone NameN2 ; \field Delta Temperature in degrees CelsiusDelta Temperature controls when mixing air from the source zone is sent to the receiving zone.; if positive, the temperature of zone from which air is being drawn (“source zone”) must be DT warmer than the zone air or no mixing occurs; if zero, mixing occurs regardless of the relative air temperatures; negative values for “Delta Temperature” are not permitted

10 Ventilation (Simple)Intent is to allow simple mechanical ventilation without specifying an HVAC system or natural ventilationAmount of ventilation determined by user defined design flow rate, schedule, and equation similar to infiltration (allows for variation based on temperature difference and wind speed)Type and control of ventilation determined by ventilation specific parameters

11 Ventilation (Simple, cont’d)Control Parameters:Minimum Temperature: indoor (zone) air temperature below which ventilation is shut offDelta Temperature: temperature differential between inside (zone) and outside air below which ventilation is shut off (negative values allowed)Advantage: can take effect of natural or simple forced ventilation into account without a lot of inputDisadvantage: user must define the air flow rate (will not figure out how much air flow there will be)Use COMIS for more serious studies of air movement within the building and between inside and outside

13 Ventilation (Simple, cont’d)A few thoughts on how to get the design flow rate for natural ventilation…ACH: consider how many air changes per hour you might expect for natural ventilation (somewhere between infiltration and fan driven flow, probably closer to infiltration)Window area and velocity: area times velocity is volumetric flow rateDetermine window opening area (not necessarily the same as window area—depends on window type)Multiply by some “standard” velocity (you will use the velocity coefficients so consider 1.0m/s)Reduce this number to account for the fact that the velocity of air at the window will not be the same as the velocity of the air at the weather station and the fact that the air must go through the buildingAdjust the temperature and wind speed parameters in the ventilation input to account for potential variations due to stack effect and/or wind effect on cross ventilation (can only estimate this without more detailed simulations)

15 Overview of the COMIS/EnergyPlus LinkCOMIS was developed in 1994 as a stand-alone multizone air flow program with its own input and output processors. In the COMIS/EnergyPlus link, COMIS is called each time step by the EnergyPlus program. Using inside and outside temperatures and the wind pressure distribution at the beginning of a time step, COMIS calculates air flows through cracks and large openings (such as open windows) between outside and inside and from zone to zone. These are then used by the EnergyPlus thermal calculation to determine surface temperatures and zone air temperatures for that time step (which are then used in the next time step to calculate new air flow values, and so on)Notes: ________________________________________________________________________________________________________________________________________________

17 Input Object DescriptionCOMIS Simulation defines basic run parameters for the air flow calculation and specifies whether wind pressure coefficients are input by the user or, for rectangular buildings, calculated by the program (New Feature for Version 1.1.1).COMIS Zone Data object specifies the ventilation control that applies to all of the openable exterior windows and doors in the corresponding thermal zone.COMIS Surface Data indicates whether a heat transfer surface (wall, window, etc.) has a crack or opening and references a COMIS Air Flow:Crack or COMIS Air Flow:Opening object that gives the air flow characteristics of that crack or opening. COMIS Surface Data can also be used to specify individual ventilation control for openable exterior windows and doors.Notes: ________________________________________________________________________________________________________________________________________________

18 Input Object Description (Cont.)COMIS Standard Conditions for Crack Data is used to normalize crack information that is based on measurements of crack air flow.If wind pressure coefficients are input by the user, COMIS Surface Data also has an associated COMIS External Node, that, via the COMIS Site Wind Conditions, COMIS CP Array and COMIS CP Values objects, gives the wind pressure distribution vs. wind direction for that node and, implicitly, for the cracks and openings in the exterior surfaces associated with that node.Notes: ________________________________________________________________________________________________________________________________________________

20 What COMIS/EnergyPlus Can DoAir flow through cracks in exterior or interzone surfacesAir flow through cracks around windows and doorsNatural ventilation, i.e., air flow through open (or partially open) exterior windows and doorsControl of natural ventilation based on inside/outside temperature or enthalpy differenceModulation of natural ventilation to prevent large temperature swingsInterzone air flow, i.e., air flow through open interzone windows and doors, and through cracks in interzone surfacesAccount for how air flow depends on buoyancy effects and wind pressureAccount for how wind pressure depends on wind speed, wind direction and surface orientationNotes: ________________________________________________________________________________________________________________________________________________

21 What COMIS/EnergyPlus Cannot DoAccount for the effect of supply-air and/or return-air flows in a zone when an HVAC air system is present and is operating. This means that the COMIS air flow simulation will give reliable answers only if there is no HVAC system, the HVAC system is off, or the HVAC system is hydronic. Air flow through cracks around windows and doors.Air circulation and/or air temperature stratification within a thermal zone. For example, you should not try to divide a high space, such as an atrium, into subzones separated by artificial horizontal surfaces that have cracks or openings with the expectation that COMIS/EnergyPlus will give you a realistic temperature in each subzone and/or a realistic air flow between subzones.Notes: ________________________________________________________________________________________________________________________________________________

22 What COMIS/EnergyPlus Cannot Do (Cont.)Bi-directional flow through large horizontal openings. See discussion below under COMIS Air Flow:Opening.Flow through ducts or other elements of an HVAC air system.Pollutant transport. There are some pollutant-related inputs but they are not used.Air-flow networks that are not connected. This means you cannot model air flow in two or more separate groups of zones.Notes: ________________________________________________________________________________________________________________________________________________

25 Correcting the Illegal COMIS Air Flow NetworkThe previous slide shows an Air-flow network that is illegal in COMIS because there are two separate groups of zones with air flow (one group is Zone-2 plus Zone-3 and the other is Zone-1). To make this legal a link (a crack or opening) between Zone-1 and Zone-2 would have to be added or the zones in one of the groups would have to be “turned off” as COMIS zones.Notes: ________________________________________________________________________________________________________________________________________________

26 New COMIS Feature Version 1.1.1 ReleaseFor rectangular buildings EnergyPlus will automatically calculate surface-averaged Cp values for the walls and roof of the building if, in COMIS Simulation, you specify Wind Pressure Coefficients = SURFACE-AVERAGE CALCULATION. In this case you do not have to enter any COMIS CP Values objects.If not calculated by program, Cp values can be obtained from wind tunnel measurements, CFD calculations, or from published values for different building shapes.Notes: ________________________________________________________________________________________________________________________________________________

33 Ventilation Control ModeVentilation Control Mode (4 types of natural ventilation control)Tout = outside air temperature Tzone = previous time step’s zone air temperature Tset = Vent Temperature Schedule value Hzone = specific enthalpy of zone air Hout = specific enthalpy of outside airTemperature: The windows/doors are opened if Tzone > Tout and Tzone > Tset and Venting Schedule allows venting.Enthalpic: The windows/doors are opened if Hzone > Hout and Tzone > Tset and Venting Schedule allows venting.Constant: Whenever Venting Schedule allows venting, the windows/doors are open, independent of indoor or outdoor conditions.NoVent: The windows/doors are closed at all times independent of indoor or outdoor conditions. Venting Schedule is ignored in this case.Notes: ________________________________________________________________________________________________________________________________________________

34 Ventilation SchedulesField: Vent Temperature ScheduleThe name of a schedule of zone-air temperature set points that controls opening of a window/door to provide natural ventilation. This schedule consists of weeks and days, with the days containing the ventilation temperature setting in ºC for each hour of the day. This ventilation temperature is the temperature above which the window/door will be opened if the conditions described under the following Ventilation Control Mode are met. [This opening control logic does not exist in the original COMIS program.]Field: Venting ScheduleThe name of a schedule that specifies when venting through this window/door is available. A zero schedule value means venting is not allowed. A value greater than zero means venting can occur if other venting control conditions (specified by Ventilation Control Mode and Vent Temperature Schedule) are satisfied. This schedule should not be confused with Vent Temperature Schedule.Notes: ________________________________________________________________________________________________________________________________________________

35 SummaryAir movement between spaces in EnergyPlus can either rely on user-defined quantities or more detailed calculationsSimple modeling statements: mixing, cross mixing, and ventilationCOMIS link provides more detailed analysis of interzone air flow as well as more sophisticated calculation of infiltration

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